Continuous recovery of high-purity Kraft lignin from black liquor via simultaneous, liquid-phase acidification and purification

A key advantage of the Sequential Liquid-Lignin Recovery and Purification (SLRP) process for the recovery of lignin from Kraft black liquor is the precipitation of a solvated, liquid-lignin phase (vs. the typical solid) during the CO2 acidification step. Here, we have discovered that this lignin can also be maintained in a solvated state during the H2SO4 acidification step if aqueous acetic acid from the Aqueous Lignin Purification with Hot Agents (ALPHA) process is incorporated into the H2SO4 solution. As the liquid-lignin is converted from its salt form at pH = 9.5 to its acid form at pH = 2.5, the lignin initially solvated with water becomes solvated and thus liquefied with the acetic acid-water solution. Maintaining the lignin in a solvated liquid state has two advantages: (1) the entire SLRP + ALPHA process, from starting black liquor to final lignin product, can be operated continuously; and (2) mass transfer of metals impurities out of the liquid-lignin phase is significantly enhanced, resulting in both a cleaner Kraft lignin product (~ 2000 ppm Na and 0.5-1.0% ash) and reduced water washing.

Automated summary

The solvated liquid-lignin phase in the Sequential Liquid-Lignin Recovery and Purification (SLRP) process can be maintained by incorporating aqueous acetic acid from the Aqueous Lignin Purification with Hot Agents (ALPHA) process, leading to continuous operation and enhanced removal of metal impurities.